1. Field of the Invention
The present invention relates to an improvement of a method for treatment of waste water, such as industrial waste water or domestic waste water, by an activated sludge process, where the organic substances contained in the waste water are decomposed with aerobic microorganisms so as to purify the waste water.
2. Description of the Related Art
Various means of purifying waste water with aerobic microorganisms have heretofore been developed and put to practical use, including a so-called activated sludge process. In the conventional apparatus for purification of waste water, the aeration tank where organic substances are decomposed with microorganisms by vigorous aeration is free from partitions therein and the aeration is effected uniformly in the aeration tank. In such a case, the pH value of the waste water to be put into the aeration tank is generally 7 or so, and it has been said that the pH value in the inside of the aeration tank is preferably slightly alkaline, e.g., from 7 to 8 or so.
On the other hand, various methods of using plural aeration tanks for various purposes have been developed. (For example, refer to Japanese Patent Application Laid-Open Nos. 54-77461, 59-39391, 60-19097 and 62-1496 and J. Ferment. Technol., Vol. 63, No. 4, pages 357 to 362, 1985.)
Further, Japanese Patent Application Laid-Open No. 58-98189, which corresponds to GB 2,112,768, illustrates a method of purification of waste water where waste water is fed into an aeration tank system, the organic substances contained in the waste water are decomposed with the microorganisms in the aeration tank and said microorganisms are separated from the drainage as drained from the aeration tank while a part of the thus separated microorganisms is re-circulated to the aeration tank system, which is composed of three tanks. The waste water and microorganisms are fed into the first tank while the amount of aeration in the second tank is made larger than that in the first tank and the amount of aeration in the third tank is made smaller than that in the first tank. In accordance with the method, the aeration tank system is divided into plural tank parts whereby the function of the microorganisms to decompose the organic substances in the plural aeration tanks is differentiated in accordance with the types of adsorption of organic substances, oxidation and decomposition of the adsorbed organic substances and restoration of the microorganisms used, so that the capacity of the microorganisms to be used in the method is enhanced and the property thereof is also improved.
However, the above method has various problems and further reduction of the aeration amount is desired in view of both the operation cost and the equipment cost. Further, a higher concentration of BOD (biological oxygen demand) and further higher load treatment thereof are desired and further improvement of the stability in purification of waste water, though relatively high, is desired.